Sound & Safe: Stevens Research Provides New Tool for Air Defense

A small jet screams across the U.S. border at top speed — its type, origin and destination unknown.

Somewhere on a remote location along that border, a microphone array mounted on a pole suddenly catches the sound of the jet's engine as it approaches, relaying it to a trailer parked just below. Instantly, a computer inside the trailer identifies the class of aircraft and transmits the information to remote operators thousands of miles away; a split-second later, a camera on the same pole pivots and begins tracking the jet by sound, snapping a few pictures that are transmitted to the operators.

Science fiction? In fact, it's already happening thanks to years of Stevens research in the field.

"Radar can tell you something is there, but it cannot detect small drones and ultralights. It cannot tell you what it is," says Hady Salloum, associate dean for research in the university’s Schaefer School of Engineering and Science. Salloum manages Stevens' development of a host of programs, including 'passive acoustic-detection' technologies and applications in diverse fields including border security and agriculture.

"This system may prove extremely useful for homeland security," he says.

Sea and sky

The system, known as Acoustic Aircraft Detection (AAD), actually has its roots under water. Literally.

The aircraft-detecting technology is built upon the same concept as the Stevens Passive Acoustic Detection System (SPADES), a patented technology previously created at Stevens and subsequently licensed to a British sound technology firm, Sonardyne.

SPADES works by placing arrays of hydrophones underwater, then acquiring and rapidly analyzing sounds to detect human divers, as well as the distinctive sounds of a variety of small boats. SPADES' software filters out ambient noise and can spot a diver breathing from a distance of hundreds of yards.

Its boat-tracking capability was honed through a series of tests in Florida, California and New Jersey, building an ever-growing library of signatures of engine and propeller frequencies to which SPADES can compare unknown vessel sounds.

"The aircraft-detection work was then a natural outgrowth of this," says Salloum, noting that the U.S. Department of Homeland Security (DHS) has supported both underwater-craft and aircraft-detection research efforts at Stevens.

For the AAD application, Stevens engineers developed new portable microphone arrays consisting of 64 tiny linked microphone capsules, each roughly the size and shape of a watch battery.

As with SPADES, a cluster of microphones transmits sound to a nearby computer, which filters out ambient noise and quickly analyzes spectrograms of target sounds of interest. The software compares those unknown sounds to proprietary libraries of aircraft-engine noises and can quickly report the ones that match.

"We are also testing the same technology so that it could potentially be used for the detection and classification of drones and unmanned aerial vehicles," Salloum adds, noting that the technology also holds potential for use at sports stadiums during practices and games to detect surveillance activity.

To build a reference library of drones' acoustic profiles, radio-controlled aircraft powered by both electric and gasoline engines were piloted above Dobbelaar Field while microphones acquired samples of their flight sounds.

Safer tires, fewer pests

Variations of the same technology have also been studied for industrial applications by Stevens researchers.

"We can use a similar technique for nondestructive evaluation of large civil structures such as bridges," says Salloum.

Other areas of acoustic research at Stevens touch upon such fields as tire damage detection and rodent and pest control.

In collaboration with a major tire manufacturer, for example, Stevens has designed a system that analyzes vibrations in order to investigate tire integrity during the manufacturing process. A hammer on a production line thumps finished tires, and the sounds of those thumps are then analyzed against a library of sounds previously acquired from damaged or defective tires.

"You can evaluate every single tire for defects, instead of every tenth tire, at very little relative cost, using this system," points out Salloum.

There is also an effort to deploy Stevens' acoustic-detection tools for rodent and pest control. Salloum came up with the idea after visiting a U.S. Customs and Border Protection facility for inspecting incoming agricultural products. Watching inspectors sort bags of rice in stainless steel bins by hand, he wondered if there might be a better way.

"Just as a diver makes a distinctive sound when he or she is breathing, insects and rodents make distinctive sounds when they are chewing, moving, mating or in distress," explains Salloum. "This is of interest worldwide, where introduced non-native pests or species can wreak havoc on native environments and food supplies. It seemed to me that there might be a match with our research."

Stevens is now investigating a system that would use microwave and acoustic technologies to detect pests in unopened containers of grain. The U.S. Department of Homeland Security agreed in May to support a test of the system, awarding a $2 million contract to the university.